Tri-Stating Brushless DC Motor Phase for Direct Detection of Back EMF Zero Cross

ABSTRACT

A back electromotive force (BEMF) zero cross may be detected in a brushless direct current (BLDC) motor that is controlled by pulse width modulation (PWM). A phase input of the BLDC motor is tri-stated during PWM periods in which the phase input conducts motor drive current, and the tri-stating of the phase input is used to determine whether a BEMF zero cross has occurred in the BLDC motor.

FIELD

The present work relates generally to controlling brushless direct current (BLDC) motors and, more particularly, to monitoring back electromotive force (BEMF) in BLDC motors.

BACKGROUND

Monitoring BEMF is a key factor in controlling BLDC motors. The zero cross point of the BEMF provides an indication of the rotor position. FIG. 1 diagrammatically illustrates a typical conventional example of a BLDC motor apparatus. The U, V and W phase inputs of the BLDC motor 13 are driven by an arrangement of drive transistors 12. A sinusoidal controller 11 provides control signals 14-19 for the drive transistor arrangement. A BEMF zero cross determiner 10 determines the BEMF zero cross point based on available feedback information. When a BEMF zero cross occurs, the determiner 10 provides a zero cross indication to an input 9 of the controller 11.

One conventional approach to BEMF determination is direct detection of BEMF using a window-opening method. In most BLDC motors, only two phases carry drive current at any time, and the third phase is floating. This opens a window to detect the BEMF in the floating winding. Window-opening has disadvantages such as relatively low efficiency and control torque distortion, and is typically ineffective in applications where noise is an important concern.

Another conventional approach is indirect estimation of BEMF using a windowless method. The windowless method makes calculations that are highly dependent on the accuracy with which the motor current is sensed. This dependency on accurate motor current sensing narrows the range of applications for the windowless method. For example, during low speed and low current operation, the motor current may be too low for accurate sensing. With core-less motors, which have very low inductance, high amplitude current ripple makes accurate current sensing even more difficult.

It is desirable in view of the foregoing to provide for BEMF zero cross determination in BLDC motors, while avoiding disadvantages associated with conventional approaches.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates an example of a BLDC motor apparatus according to the prior art.

FIG. 2 is a timing diagram that illustrates conventional operation of a BLDC motor.

FIG. 3 is a timing diagram that illustrates principles of the present work applied with respect to the operation shown in FIG. 2.

FIG. 4 diagrammatically illustrates a BLDC motor apparatus according to example embodiments of the present work.

FIGS. 5 and 6 illustrate operations performed according to example embodiments of the present work.

DETAILED DESCRIPTION

FIG. 2 is a timing diagram that illustrates a conventional example of low speed operation of a low inductance (e.g., core-less) BLDC motor. The U, V and W phases are controlled according to sinusoidal control that employs a series of pulse width modulation (PWM) periods as shown. The BEMF zero cross is expected in phase U when the voltage of phase W is zero. In the FIG. 2 example, the U and V phase voltages have approximately 5% and 10% duty cycles, respectively, relative to the PWM period length. The present work recognizes that the phase U current is approximately zero for about 90% of the PWM period, and exploits this characteristic for BEMF zero cross detection.

FIG. 3 is a timing diagram that illustrates principles of the present work applied with respect to the example operation of FIG. 2. Phase U is tri-stated temporarily during each PWM period. The resulting voltage on floating phase U (shown by broken line) may then be compared to the motor ground (GND) to detect the BEMF zero cross point directly. That is, from one PWM period to the next, if the voltage on floating phase U crosses from above/below GND to below/above GND, this indicates a BEMF zero cross. The temporal duration of the phase U tri-stating is designated as a “slot” in FIG. 3. In each PWM period, the slot begins approximately at a mid-point 31 of the period, and ends at a point 32 before the period ends, that is, before the (10% duty cycle) phase V voltage is driven high. Because the phase U current is near zero for almost all of the PWM period, it will be understood that various embodiments use various temporal configurations of the slot (i.e., various combinations of slot length and slot location).

FIG. 4 diagrammatically illustrates a BLDC motor apparatus according to example embodiments of the present work. The apparatus of FIG. 4 is capable of the above-described operation shown in FIG. 3. In some embodiments, the apparatus includes a controller 11, drive transistor arrangement 12 and BLDC motor 13 such as described above and shown in FIG. 1. The apparatus further includes a tri-state controller shown at 40-44, and a zero cross determiner shown at 45-49. The tri-state controller includes an AND gate 40 having an input driven by the control signal 14 that normally controls transistor 21 of the drive transistor arrangement (see also FIG. 1). The other input of AND gate 40 is driven by the inverse of a signal 43 output from another AND gate 44 whose inputs are driven by a slot enable signal 41 and a slot signal 42.

Referencing also FIG. 3, in each PWM period, the slot signal 42 is active (high) from time 31 to time 32, and is otherwise inactive (low). The slot enable signal 41 is active (high) when a motor speed command of the BLDC motor apparatus is less than a predetermined threshold, e.g., 50% in some embodiments, and is otherwise inactive (low). For motor speed commands above the threshold, some embodiments use conventional techniques (e.g., the BEMF zero cross determiner 10 of FIG. 1) to determine the BEMF zero cross. When the slot enable signal 41 qualifies the slot signal 42 at AND gate 44, signal 43 goes high and the output 14′ of the AND gate 40 is therefore driven low. This turns off transistor 21 to tri-state the phase U input of BLDC motor 13. Note that the aforementioned inversion of the output 43 of AND gate 44 results in a NAND gate between AND gate 40 and the signals 41 and 42.

The phase U motor input is coupled to one input of a comparator 45 whose other input is coupled to the motor ground GND, which serves as a reference voltage for the comparator 45. A sampler 47 samples the output 49 of comparator 45 while the phase U motor input is tri-stated (i.e., while the slot signal 42 is active). The comparator output 49 is a non-zero voltage only when the phase U voltage exceeds the reference voltage GND. Thus, in every PWM period, the sampler 47 receives a comparator output voltage (compare result), and produces either a sample value of 1, if the voltage is non-zero, or a sample value of 0, if the voltage is zero. The sampler 47 therefore outputs either a 1 or 0 sample value for every PWM period, resulting in a series of is and 0s at the sampler output 46. A change detector 48 receives this series of is and 0s, and detects 0-to-1 changes and 1-to-0 changes in the series. Any such change corresponds to a BEMF zero crossing, which change detector 48 signals to the input 9 of the controller 11.

FIG. 5 illustrates operations according to example embodiments of the present work. The apparatus of FIG. 4 is capable of performing the operations shown in FIG. 5. At 51, phase U is tri-stated during the current PWM period. At 52, the tri-stated phase U is compared to GND, and the compare result is sampled at 53. The tri-stating ends at 54 before the current PWM period ends. The operations at 51-54 are then repeated during the next PWM period, as indicated at 55.

FIG. 6 illustrates further operations according to example embodiments of the present work. The change detector 48 of FIG. 4 is capable of performing the operations shown in FIG. 6. At 61, the current sample value (see also FIG. 5) is compared to the previous (i.e., immediately preceding) sample value. If the compared sample values are determined to differ from one another at 62, then a zero cross is indicated at 63. Otherwise, the next sample value is awaited at 64, after which the operations at 61-63 are repeated.

The BEMF zero cross detection described above relative to FIGS. 3-6 has, among others, the following advantages over prior art techniques: does not distort control torque; eliminates dependency on current sensing/measurement; eliminates calculations based on current sensing; eliminates dependency on position sensors (e.g., Hall elements); provides improved noise performance; provides improved speed stability; provides improved frequency jitter characteristics; provides improved zero cross detection at low motor speeds and low motor currents; and provides improved zero cross detection in core-less motors.

Although example embodiments of the present work have been described above in detail, this does not limit the scope of the work, which can be practiced in a variety of embodiments. 

What is claimed is:
 1. An apparatus for controlling a brushless direct current (BLDC) motor, comprising: a drive circuit configured to drive phase inputs of the BLDC motor; a controller coupled to said drive circuit and cooperable therewith for controlling the BLDC motor by pulse width modulation (PWM); a tri-state controller coupled to said drive circuit and cooperable therewith for tri-stating one of the phase inputs during PWM periods in which said one phase input caries motor drive current; and a zero cross determiner coupled to said drive circuit and configured to use said tri-stating of said one phase input to determine whether a back electromotive force (BEMF) zero cross has occurred in the BLDC motor.
 2. The apparatus of claim 1, wherein said zero cross determiner includes a comparator for comparing said one phase input, while tri-stated, to a reference voltage to obtain a compare result.
 3. The apparatus of claim 2, wherein said zero cross determiner includes a change detector coupled to said comparator and configured to detect when said compare result differs from a previous said compare result obtained during an immediately preceding said PWM period.
 4. The apparatus of claim 3, wherein a BEMF zero cross is determined if said compare result differs from said previous compare result.
 5. The apparatus of claim 2, wherein the reference voltage is a ground of the BLDC motor.
 6. The apparatus of claim 1, wherein said tri-stating occurs while the motor drive current in said one phase input is approximately zero.
 7. The apparatus of claim 6, wherein said tri-stating commences approximately at a mid-point of the PWM period and ends before the PWM period ends.
 8. The apparatus of claim 6, wherein said tri-stating occurs during a latter half of the PWM period.
 9. The apparatus of claim 1, wherein said controller is a sinusoidal controller and the BLDC motor is core-less.
 10. A method of detecting a BEMF zero cross in a BLDC motor controlled by PWM, comprising: tri-stating a phase input of the BLDC motor during PWM periods in which the phase input carries motor drive current; and using said tri-stating of said one phase input to determine whether a BEMF zero cross has occurred in the BLDC motor.
 11. The method of claim 10, wherein said using includes comparing said one phase input, while tri-stated, to a reference voltage to obtain a compare result.
 12. The method of claim 11, wherein said using includes detecting when said compare result differs from a previous said compare result obtained during an immediately preceding said PWM period.
 13. The method of claim 12, wherein a BEMF zero cross is determined if said compare result differs from said previous compare result.
 14. The method of claim 11, wherein the reference voltage is a ground of the BLDC motor.
 15. The method of claim 10, wherein said tri-stating occurs while the motor drive current in said one phase input is approximately zero.
 16. The method of claim 15, wherein said tri-stating includes commencing said tri-stating approximately at a mid-point of the PWM period and ending said tri-stating before the PWM period ends.
 17. The method of claim 15, wherein said tri-stating occurs during a latter half of the PWM period.
 18. The method of claim 10, wherein the BLDC motor is core-less and controlled in accordance with a sinusoidal control.
 19. A BLDC motor apparatus, comprising: a BLDC motor having a plurality of phase inputs; a drive circuit coupled to said phase inputs for driving said phase inputs; a controller coupled to said drive circuit and cooperable therewith for controlling the BLDC motor by PWM; a tri-state controller coupled to said drive circuit and cooperable therewith for tri-stating one of the phase inputs during PWM periods in which said one phase input carries motor drive current; and a zero cross determiner coupled to said drive circuit and configured to use said tri-stating of said one phase input to determine whether a BEMF zero cross has occurred in the BLDC motor.
 20. A BLDC motor apparatus, comprising: a BLDC motor having a plurality of phase inputs; a transistor drive circuit coupled to said phase inputs for driving said phase inputs; a sinusoidal controller coupled to said transistor drive circuit and cooperable therewith for controlling the BLDC motor by PWM; a tri-state controller coupled to said transistor drive circuit and cooperable therewith for tri-stating one of said phase inputs during PWM periods in which said one phase input carries motor drive current but while the motor drive current is approximately zero; and a zero cross determiner coupled to said sinusoidal controller and said transistor drive circuit, said zero cross determiner configured to use said tri-stating of said one phase input to determine, and indicate to said sinusoidal controller, whether a BEMF zero cross has occurred in said BLDC motor; wherein said tri-state controller includes a NAND gate having an enable input and a further input for receiving a first control signal driven active approximately at a mid-point of the PWM period and driven inactive before the PWM period ends, and an AND gate having an input that receives an output of said NAND gate and a further input for receiving a second control signal from said sinusoidal controller, and wherein said AND gate has an output that, when said first control signal is active, controls a transistor in said transistor drive circuit to tri-state said one phase input; wherein said zero cross determiner includes a comparator having an input coupled to said one phase input, a further input coupled to a ground of said BLDC motor, and an output that produces a compare result voltage while said one phase input is tri-stated, a sampler coupled to said output of said comparator and configured for sampling said compare result voltage to obtain a sample value, and a change detector coupled to said sampler and configured for detecting when said sample value differs from a previous said sample value associated with a previous said compare result voltage produced during an immediately preceding said PWM period, and further coupled to said sinusoidal controller for providing a zero cross indication to said sinusoidal controller when said sample value differs from said previous sample value. 